1. Field of the Invention
The present invention relates to a circular slide rule apparatus which may be used by recreational divers to enhance their safety while simultaneously increasing the amount of time permitted for their dives, and to provide a systematic procedure for operating at more than one depth during a single dive.
A risk of diving with a Self Contained Underwater Breathing Apparatus (SCUBA) is the development of decompression sickness (hereafter called DSC, but commonly called "the bends"). DCS is a general term that includes a number of signs and symptoms whose common etiology is the formation of inert gas bubbles in one or more parts of the body and whose most frequent manifestations are joint pain and central nervous system damage, ranging in severity from temporary mild discomfort to sudden death. While many different gas combinations are used in military and commercial diving, recreational divers typically breathe comprssed air, which is 79% nitrogen, and nitrogen is the inert gas relevant to this discussion.
A recreational diver at depth breathes air at a pressure greater than at the surface and becomes of this gradient, nitrogen passes through the lung membranes into solution in the blood, and from there into the various tissues of the body, at varying rates that depend upon the magnitude of said pressure gradient and upon characteristics of each of said various tissues. When the diver returns to the surface, the reverse process occurs: the higher pressure nitrogen dissolved in the blood passes through the lung membranes and is exhaled, and as the nitrogen pressure in the blood diminishes, dissolved nitrogen passes from the tissues to the blood until eventually all portions of the body have reached equilibrium with atmospheric pressure; stated simply, pressure changes flow from high to low.
If a diver stays at a given depth of water long enough for any of his tissue pressures to exceed the limit at which it becomes impossible to undergo this normal process of gas elimination, and said tissue pressure is sufficiently greater than that of the surrounding environment, then nitrogen bubbles can form in a body tissue and that diver could develop DCS. To minimize this risk, the diver must either avoid such depth and time combinations, or must undergo the procedure known as decompression, which requires staying at predetermined depths for predetermined time periods so as to eliminate the dangerous excess of nitrogen pressure. In recreational diving, decompression is considered to be an emergency procedure only, for use when well known time/depth limits, or no-decompression limits, have been accidentally exceeds. Because of the dangers associated with the procedure, it is not considered to be an elective option.
If a diver planned only a single dive to a single depth, the requirements would be simple: the diver should never exceed the empirically determined no -decompression limit for that depth. However, it is commonplace for divers to perform two or more dives in succession, and the excess nitrogen in the body cannot reach equilibrium with the atmosphere in the short time span(s) between dives. Thus, a diver would reenter the water with a tissue nitrogen pressure greater than atmospheric, and must allow for said greater pressure in computing the maximum time permissible at depth during a subsequent dive. To avoid decompression procedures, a satisfactory adjustment factor must be employed. It is therefore essential that the recreational diver have a means instantly available at all times to compute with speed, ease, and precision the amount of time which he can spend at any depth without exceeding the limit of time for that depth, and if more diving is planned within a short period, he/she must alsod be able to calculate the amount of nitrogen pressure lost during the time spent on the surface between dives, so that the net accumulation and loss of nitrogen pressure in the body tissues may be safely and accurately tracked over a succession of dives.
2. Prior Art
Studies relating to the hazards of diving go back for centuries. There is a very extensive body of literature dealing with the theoretical model of decompression developed by the British physiologist J. S. Haldane in his landmark paper of 1907, which is the foundation of hyperbaric (greater than atmospheric pressure) physiology, and which first provided decompression information in tabular format, commonly called "tables", to sever as a safety guide for divers and caisson workers. Many different tables have been developed over the years, new tables appear periodically, research is ongoing, and more tables may be expected. Since virtually all tables ever constructed are based ultimately on Haldane's work, a brief discussion of a relevant portion of it is appropriate. (The English system of measurements is used herein, but metric values could easily be employed by making the appropriate conversions.)